1. Field of the Invention
This invention relates to voice communications over distributed digital networks (e.g. networks which do not have a central station) such as local area networks and, more particularly, to interfaces between such distributed digital networks and analog devices such as telephone sets.
2. Description of the Prior Art
Telephonic communications have been well known and in widespread use for many years. The interconnection arrangements which allow selective connection of telephone sets is complex and has developed from manually operated switchboards to elaborate central switching systems now in commercial use. The use of telephonic communication is central to the efficient operation of many offices, laboratories, businesses and the like. Further developments in other fields have also been adapted for use with telephonic communications systems, permitting communication of data other than voice over the telephone system, such as facsimile messages and data originating in digital form in data processing equipment. More recently, telephonic communication systems have begun adopting digital transmission techniques allowing improved noise immunity and greater data density on the existing communication links. The use of optical fibers for the communication link is also an excellent medium for transmission of numerous digital signals representing acoustic or voice signals. However, use of commercial digital voice transmission systems has generally been limited to long distance communications and analog to digital conversion is done at a central office which interfaces with the long distance links. Therefore the analog connection from the subscriber telephone set to the central station remains subject to noise and interference problems.
Similarly, development of data processing equipment has resulted in the use of local, wide and global area networks for transmission of digital data between data processing systems and which will be collectively referred to herein as distributed digital networks, allowing more rapid transmission than is possible over analog telecommunication links using modulator/demodulator devices (Modems) to convert digital signals into audio frequency tones for transmission. These distributed digital networks are also distinguished from telecommunication networks by the fact that they do not have a central station. Thus control of the network, as would otherwise be exerted by a central station, is "distributed".
A distributed digital network has several inherent advantages over a telecommunication network having a central station. Most importantly, a distributed digital network will have no single point of failure and communications may continue between functional stations even when one or more of the stations is non-functional. Further, the distributed digital network can be arranged to enhance security and reliability of communications between stations on the network, such as by encryption or the increase of resistance to interference. A distributed digital network need not be limited to subscriber stations on the network since inter-network or gateway interfaces can be provided to allow connection of the distributed digital network to another network, such as a commercial network.
Distributed network systems having digital protocols are known which include arrangements for communication by devices other than data processing equipment such as facsimile equipment, bulk storage devices, data bases, acoustic couplers, printers, mainframe access units and digital telephones. U.S. Pat. No. 4,697,263 to Beaufoy is exemplary of such arrangements. However, as disclosed therein, since there is no common equipment on such a network, each device connected to the network must contain its own clock and complex synchronization schemes are necessary to permit reliable transmission and reception of data. Such systems also require compatibility between equipment connected to the network, even though interfaces are used which may be particularized to the equipment and, as disclosed in the above noted patent, "dedicated special purpose stations" are required for special functions such as telephone conferencing, short code dialling, external connection to other communication networks, voice recording, network usage accounting and protocol conversion.
With regard to voice communication over the system of Beaufoy, the generation of call progress tones or so-called supervisory tones to indicate call status presents a substantial burden and it is indicated to be preferable that a display be substituted to avoid the need to produce such tones. If such supervisory tones are required, it is suggested that they may be generated by the microprocessor which also must control the time slot processor and which is subject to interrupts by the time slot processor. As disclosed therein, the time slot processor monitors each time slot and generates an interrupt signal upon a match of the address portion of a data packet with the station address.
Nevertheless, the use of supervisory tones is well established in the telecommunications art and is heavily relied upon by users in operating telephone equipment in a manner which has come to be natural to such users. Therefore, it is extremely desirable that the communication over a digital network without a central switching facility, which would otherwise generate such supervisory tones, be entirely transparent to the user. It is also important to provide an arrangement allowing call forwarding and conference calling in a manner which is similarly transparent to the user. Moreover, since the use of supervisory tones has been such a long standing convention in the telecommunication system art, other devices which are capable of communicating at audio frequencies over the phone lines will often use some supervisory tones for control functions. Therefore, a digital network which is transparent to the user eliminates the need for specialized equipment. For instance, standard telephone sets, which term as used herein includes telephone sets adapted to be used by the handicapped (where the existence of, say, a dial tone or busy signal may be displayed to the user visually or tactilely), acoustically coupled modems, answering machines, encryption devices, facsimile machines and the like which are usable with commercial telecommunications networks, could also be used with such a system in accordance with the invention.
By the same token, it is desirable that there be no requirement that interface to the network be particularized to the network on which the voice communication is provided and that additional synchronization problems, beyond those already provided for in the network itself, be minimized or eliminated altogether. Specifically, it is desirable that the voice transmission capability not cause additional constraints on the network design or operation or require particular circuitry which is adapted for a particular network. It is particularly desirable in this regard that a voice communication arrangement for communicating over a distributed digital network, such as a local area network, be at least as, if not more, fault tolerant than the digital data processing devices which the network was originally designed to serve.